In “The Party,” British filmmaker Sally Potter examines the state of politics and relationships in her country through a darkly comic story in which seemingly civil people lose their couth and behave terribly during a social gathering. This is a film you hope will delight the edgier desires of your funny bone. But it is too sketchy and stagy to achieve that, despite effective work from its terrific cast.

Potter is an adventurous filmmaker whose credits include the gender-bending “Orlando” and the risky “Yes,” which was written in iambic pentameter. Like her 2013 coming-of-ager, “Ginger and Rosa,” her new film features left-leaning characters who espouse admirable principles but conduct themselves rottenly in private.

Presented in 1960s-inspired black and white, the film is a disastrous-party farce, like “Who’s Afraid of Virginia Woolf?” or “The Discreet Charm of the Bourgeoisie.”

In her London town house, career politician Janet (Kristin Scott Thomas) is hosting an intimate dinner party to celebrate her ascension to a health-minister post within her unnamed liberal opposition party. Busy and cheery, Janet prepares food and handles calls from well-wishers and a secret lover.

In the living room, meanwhile, Janet’s academic husband Bill (Timothy Spall) sits alone, soused and gloomy, listening to an old blues LP. Is he less supportive of Janet’s success than Janet believes?

It is Bill, however, who makes the most shocking announcement, and it triggers an eruption of disclosures and confessions. Insults fly. Relationships crumble. The overall tempest includes betrayals, an illness, lots of arguing, some ruined canapes and a gun.

Potter’s screenplay has an IQ, and in delivering her better one-liners and embodying her conception of the movie as a “comedy wrapped around a tragedy,” her actors enliven the picture and convey shades in their potentially cartoonish characters.

Scott Thomas’s Janet impressively combines caring with thoughtlessness. Spall’s Bill radiates pent-up resentment. Clarkson’s April issues nasty remarks with comically scary ease, assisted by Potter’s having given her the best lines (a comment about aromatherapy and Nazis comes to mind).

But unfortunately, the winning moments don’t add up to a compelling whole.

Potter’s decision to present her story like a filmed play undermines emotional impact.

The vagueness with which Potter defines her characters’ ideologies impedes our ability to grasp her screenplay’s many political aspects.

Compactly edited, “The Party” isn’t the worst way to spend 71 minutes of viewing time. But given the talent involved, it disappoints.